\(a,\dfrac{12}{15}+\dfrac{13}{15}\\ =\dfrac{12+13}{15}\\ =\dfrac{25}{15}\\ =\dfrac{5}{3}\\ b,\dfrac{25}{35}-\dfrac{18}{35}\\ =\dfrac{25-18}{35}\\ =\dfrac{7}{35}=\dfrac{1}{5}\\ c,\dfrac{9}{5}+\dfrac{3}{4}=\dfrac{36}{20}+\dfrac{15}{20}\\ =\dfrac{36+15}{20}\\ =\dfrac{51}{20}\\ d,\dfrac{18}{12}-\dfrac{8}{6}\\ =\dfrac{18}{12}-\dfrac{16}{12}\\ =\dfrac{2}{12}\\ =\dfrac{1}{6}\\ đ,3+\dfrac{2}{3}\\ =\dfrac{3}{1}+\dfrac{2}{3}\\ =\dfrac{9}{3}+\dfrac{2}{3}\\ =\dfrac{11}{3}\\ e,3-\dfrac{1}{3}\\ =\dfrac{3}{1}-\dfrac{1}{3}\\ =\dfrac{9}{3}-\dfrac{1}{3}\\ =\dfrac{8}{3}\\ i,\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}\\ =\dfrac{6}{12}+\dfrac{4}{12}+\dfrac{3}{12}\\ =\dfrac{13}{12}\\ g,\dfrac{9}{2}-\dfrac{1}{3}-\dfrac{2}{4}\\ =\dfrac{54}{12}-\dfrac{4}{12}-\dfrac{6}{12}\\ =\dfrac{44}{12}\\ =\dfrac{11}{3}\)
Bài 4:
Mẫu là (120+20)/2=140/2=70
Tử là 70-20=50
